Method for manufacturing cement clinker

ABSTRACT

The invention relates to a method for manufacturing cement clinker whereby the raw meal is initially subjected to preparatory processing, e.g. by comminution, homogenization and/or drying, and, eventually, nodulized and burned, with the nodulization process itself taking place in a stationary burning reactor. By the method according to the invention a sulphatic compound is added to the raw meal in a sufficient quantity before introducing the raw meal into the stationary burning reactor or directly into the stationary reactor, e.g. through the combustion air or together with the fuel. The sulphatic compound may be selected among all types of sulphates which will not affect the properties of the finished cement. It is particularly advantageous to use calcium sulphate, either naturally occurring or industrial by-products and regular waste materials, e.g. used absorbent from dry exhaust gas desulphurization. In order that the method according to the invention can be carried out it is preferred that the sulphate content of the calcined raw meal be considerably higher than the alkali oxide content, in accordance with the following formula (the percentages indicated are percentages by weight): 
     
         %SO.sub.3 &gt;%K.sub.2 O+1.5·%Na.sub.2 O 
    
     where the SO 3  percentage represents the total content of sulphate in the material streams introduced into the process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing cementclinker by which method the raw meal is subjected to preparatoryprocessing, e.g. by comminution, homogenization and/or drying,whereafter the raw meal is preheated and calcined and, eventually,nodulized and burned, and where nodulization takes place in a stationaryburning reactor.

A stationary burning reactor is defined as a reactor with stationarywalls as opposed to a rotary kiln. The most prevalent types ofstationary reactors are fluid bed or spouted bed reactors.

Nodulization and clinkerization take place when the preheated andpossibly calcined cement raw meal, subject to simultaneous input of heatand a consequential formation of a melting phase, is transformed from aparticle-formed material with a fineness which is typically less than15% in excess of 0.090 mm into a grained or ball-shaped material with afineness of more than 80% in excess of 0.5 mm.

In the general cement clinker manufacturing process, the formed meltingphase is an oxide fusion which is formed at a temperature ofapproximately 1300° C. and with a composition which is roughly asfollows: 55% CaO, 6% SiO₂, 23% Al₂ O₃ and 17% Fe₂ O₃, and where theoxide fusion will normally constitute 15-25% of the total clinker mass.

In the vast majority of existing manufacturing plants, clinkerizationtakes place in a rotary kiln. Given that the costs of construction andthe expenditure relating to the maintenance of the rotary kiln are veryhigh because of the moving parts, numerous attempts have been made toeliminate the need for the rotary kiln in the cement manufacturingprocess.

2. Description of the Prior Art

It is known practice, e.g. from the published German patent applicationNo. 42 19 697, to manufacture cement clinker according to a method wherethe burning of the calcined material is effected in two stages and wherethe first stage consists of a fluid bed or spouted bed. In this firststage the raw meal is heated to a presintering temperature, at which amelting phase is formed, and clinkerization is initiated once themelting phase is present. The mate-rial is subsequently directed to thesecond stage where the clinker is eventually burned at a temperature ofminimum 1300° C.

However, practical implementation of this method is difficult due to thefact that the melting phase formed occurs within a range of a fewdegrees, hence making it extremely difficult to maintain the processconditions necessary to ensure constant presence of a predetermined,small amount of melting phase. This may, in particular, pose problemssince the amount of fused material depends to a major degree on thecomposition of the raw meal, and not just on the temperature.

In the U.S. patent application Ser. No. 2.465.420 a process is describedfor calcination and nodulization of calcareous sludge using a saltfusion of alkali metals. Such a salt fusion is not always suitable forthe manufacture of cement since an excessively high content of alkalimetals in the finished cement may give rise to the formation ofexpanding reaction products, which will lead to cracks in the finishedconcrete.

In Danish patent application No. 1579/85 is described a method fortreatment of a used absorbent from a dry exhaust gas desulphurizationprocess. Since, according to the method described in this application,presence of a certain amount of chlorides is a precondition, it wouldnot be advantageous to use this method for manufacturing cement clinkersince chlorides may cause operational problems in the form of cakings inpreheater and calciner.

It is prior art to add to the raw meal for manufacturing cement up toabout 5% gypsum together with fluorine in order to lower the temperatureat which C₃ S is formed, and hence also the burning temperature, but upto this point in time the nodulization and burning of these cements havebeen carried out in rotary kilns.

SUMMARY OF THE INVENTION

By means of this invention a method is thus indicated for themanufacture of cement which will eliminate operational problems and theaddition of unwanted substances, such as chlorides, while, at the sametime, the nodulization and the burning of the clinker can be carried outin a stationary reactor, in which there are specific requirements withrespect to the controlling of fused material formation.

According to the invention, this is achieved by addition of a sulphaticcompound to the raw meal in a sufficiently large quantity prior to theintroduction of the raw meal into the stationary burning reactor or byaddition of a sulphatic compound directly into the stationary reactor,e.g. through the combustion air or together with the fuel.

It is surprising that the addition of a sulphatic compound, which doesnot necessarily itself have to be capable of melting, can cause amelting phase to be formed in the material in the stationary reactor andthat the melting phase which is formed satisfies the requirement that itmust be possible to regulate the extent of the melting phase bycontrolling the temperature. However, a temperature range exists between1000°-1300° C. in which the added sulphate compound may be mixed withthe natural content of alkali metals of the cement raw meal, forming afusion which essentially consists of the added sulphate compound and asmaller quantity of alkali sulphates (K₂ SO₄ and Na₂ SO₄).

In principle, the sulphatic compound may be selected among all types ofsulphates which do not have any adverse effect on the quality of thefinished cement. It would be particularly advantageous to use calciumsulphate, either naturally occurring or industrial by-products andregular waste materials, e.g. in the form of used absorbent from dryexhaust gas desulphurization. If the raw meal itself has a low contentof alkali metals, a certain amount of the sulphate source can be addedas alkali sulphate. Also, the sulphate compound may, to a certainextent, be introduced together with the fuel.

It is a distinctive feature of this fusion that its quantity variesevenly with the temperature and that, to a major degree, it isindependent of the sulphate content, as long as this content issufficiently high.

Optimum temperature conditions for nodulization of the calcined raw mealby means of this fusion exist within the temperature range 1100°-1250°.

In order to carry out the method according to the invention it is anessential requirement, as mentioned in claim 1, that the sulphatecontent of the calcined raw meal is considerably higher than the alkalioxide content. It is preferable to ensure compliance with the followingformula (the percentages indicated are percentages by weight):

    %SO.sub.3 >%K.sub.2 O+1.5.%Na.sub.2 O

where the SO₃ percentage is the total content of sulphate in thematerial streams into the process.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further details in the following withreference to the drawing which shows an example of a plant in which themethod according to the invention can be used. This plant is, subject tofew modifications, known from European patent No. 0380878 (Dec. 14,1989, F.L. Smidth & Co., A/S).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the FIGURE is shown a plant for manufacturing of cement whichcomprises a preheater which consists of three preheater cyclones 1, 2and 3, a calciner 4 with a separation cyclone 5, a stationary reactor 6,which is located below the calciner 4 and a unit 7 in which the finalreaction of the clinker can take place, and a clinker cooler 8. The unit7 may e.g. be a fluid bed chamber or a short rotary kiln.

In operation, cement raw meal is fed to the plant through an inlet 9 andthen conveyed in known manner through the preheater cyclones 1, 2 and 3to the calciner 4 via a duct 10. The calciner 4 is fed with fuel via aninlet 11 and the sulphatic compound through an inlet 11a. Combustion airis added through one or several ducts 12 from the clinker cooler 8.

In the calciner 4 the preheated raw meal is calcined in gas suspensionand the suspension of exhaust gas and calcined raw meal is subsequentlydirected via an outlet 5a to the separation cyclone 5 from which theexhaust gas continues up through the preheater 1, 2 and 3, before it iseventually discharged through an outlet 13. The calcined raw meal isdirected from the separation cyclone 5 down into the reactor 6 via aduct 14.

The reactor 6 is provided with combustion air from the unit 7 through aduct 15 and with fuel via an inlet 16. The unit 7, to which the finishedclinker is directed, is supplied with air and fuel to an extentnecessary to allow the final reaction of the clinker to take place in anappropriate environment. Finally, the clinker passes via a duct 18 intothe cooler 8.

In the case where the reactor 6 is a spouted bed, the calcined raw mealwill, when it is introduced via the duct 14, flow down along the conicalwalls in the lowermost part of the reactor 6. When the raw meal reachesthe bottom of the conical part, the small particles will be entrained inthe upward-flowing combustion air which is supplied via the duct 15.This will cause a spouted bed to be formed, in which the material iscirculated in a characteristic pattern, with the material beingconverted into nodules with constantly increasing particle dimensions.

When the particles have attained a weight which is such that the pull ofthe nodules exerted by the gravitational force is greater than the forcewhich is generated by the upwardflowing gas, the finished nodules falldown through the conical part of the reactor and down through the duct15 to the unit 7. In the unit 7, the generated nodules will have thenecessary time to react so that the desired clinker minerals are formed.The size of the finished nodules depends on the gas volume which isdrawn through the duct 15, and hence on the velocity of the gas.

A test was carried out during which calcined raw meal was granulated inthe reactor 6 without additional input of sulphate, exclusively usingthe raw meal's own oxide fusion. At a temperature of about 1325° C. itwas possible to produce nodules, but it was unavoidable that asubstantial part of the material was accumulated on the sides of thereactor chamber to such an extent that operation was not possible formore than 5 hours. This result is attributed to an excessive quantity offused material during the granulation process.

In a second test, the temperature was significantly reduced to about1200° C. In the event, it emerged that the granulation process wasbrought to an almost complete standstill, with only sporadic quantitiesof nodules being discharged from the reactor 6. Instead, a violentcircuit of material was established between calciner 4, separationcyclone 5 and the reactor 6.

In a third test, 5 kg of finely ground gypsum was added per 100 kg rawmeal to the calciner 4 via the inlet 11a and the temperature in thereactor 6 was set for 1200° C. After a certain time period, anequilibrium was established in the circuit between calciner 4,separation cyclone 5 and reactor 6, with granular material beingdischarged from the reactor through 15 in a flow stream equivalent tothe input of raw materials at 9 and 11a. The manufactured cement clinkerhad a mean particle size of 3 mm.

I claim:
 1. A method for manufacturing Portland cement clinkercomprising the steps of:(1) drying and preheating cement raw material;(2) calcining the dried, preheated raw material in a calciner; (3)burning the calcined raw material in a stationary walled reactor chamberat a temperature between 1000°-1300° C.; (4) adding to the cement rawmeal during the drying, preheating, calcining or burning steps aquantity of sulfate compound sufficient to cause the formation of amelting phase; (5) controlling the quantity of said melting phase byadjusting the temperature in the calciner or reactor, such thatclinkerized product is withdrawn from the reactor in the form of noduleswith a fineness of more than 80% in excess of 0.5 mm, and withoutsubstantial accumulation of material on the walls of the stationaryreactor chamber.
 2. A method according to claim 1 wherein thetemperature in the reactor is maintained at 1100° C. and 1250° C.
 3. Amethod according to claim 1 wherein the amount by weight of sulfate inthe raw material is greater than the combined percent by weight of K₂ Oplus 1.5 times the percent by weight of Na₂ O.
 4. A method according toclaim 1 wherein the sulfate is calcium sulfate.